This invention relates to measurement apparatus for antenna patterns in general and more particularly to a near field measurement technique for providing a boresight error.
A boresight only measurement facility is a special class of near field measurement systems in which the primary manifestation is in the measurement of the phase distribution of a near field pattern. An antenna system with zero boresight error with respect to the near field measurement plane would have a constant phase throughout the collimated region of the near field for the sum mode and 180 degree step function for the difference mode. Thus a boresight error manifests itself as a tilt of the phase front and can be determined by a mathematical relationship.
Essentially, the boresight error is the difference between an assumed boresight direction or a mechanical boresight direction and the electrical boresight direction determined from the radiation of the antenna. The use of near field measurement techniques for field antenna pattern determination was developed many years ago. The near field measurement technique became practical over 20 years ago with the advent of phase/amplitude receivers and with the development of the theoretical basis for probe compensated measurements, sampling and digital computation. The near field measurement techniques have been accepted in the industry and have been used by the military for measuring antenna patterns. The technique has become the NBS standard for gain calibration of microwave antennas.
Near field measurement systems are used not only to determine far field patterns of antennas but are also used to detect fault elements of phased array antennas, phase align phased antennas, focus antennas and boresight antennas. Many of these require the use of only the near field data without the need for far field calulation.
Essentially, all measurement techniques for measuring the far field radiation pattern of an antenna can be used to measure the boresight error of an antenna. These techniques include the elevated outdoor far field range, the ground reflection far field range, the slant far field range, the anechoic chamber far field range, point source compact range, the line source compact range, point source lens range, the planar surface near field range, cylindrical surface near field range and spherical surface near field range. Each of these ranges is capable of complete far field antenna pattern measurement and thus boresight measurement. These techniques as indicated above, require extensive equipment including expensive antenna mounts, receivers pattern recorders and are specifically designed for high accuracy boresight measurement. There are, of course, existing in the prior art several apparatuses specifically designed for high accuracy boresight error measurement which include the far field difference pattern null seeker, far field sum pattern beam straddler and point source, lens assembly. The first two techniques are adapations of the far field antenna pattern measurement technique and thus require extensive equipment and normally a large outdoor or anechoic chamber facility. The third is an adaptation of the point source compact lens technique.
In any event, a near field measurement system has great advantages when compared to other techniques for measuring the boresight error. In such a system one can achieve high accuracy in a relatively small size with an attendant increase in reliability and utilizing accepted techniques. As will be explained, the present system is a near field measurement system which is fully automated and is capable of performing many measurements in a short time. The system can measure a phase distribution with approximately 30 near field measurements in one minute.